The cardiac conduction system is the heart’s built-in electrical network that coordinates each heartbeat. It makes the atria contract first, then the ventricles, so blood moves efficiently through the heart and into the body. This timing matters because even small delays or misfires can reduce blood flow, cause palpitations, or create dangerous arrhythmias.
Doctors use the electrical pattern of the heart to check whether this system is working normally.
The signal normally begins in the sinoatrial node, spreads across the atria, pauses briefly at the atrioventricular node, then travels through the bundle of His, bundle branches, and Purkinje fibers. The AV node delay gives the ventricles time to fill before they contract. An electrocardiogram, or EKG, records voltage changes on the skin that reflect atrial depolarization, ventricular depolarization, and ventricular repolarization.
By reading wave shapes and time intervals, clinicians can identify rhythm problems, conduction blocks, and signs of heart stress.
Key Facts
- Normal electrical path: SA node to atria to AV node to bundle of His to right and left bundle branches to Purkinje fibers.
- The SA node is the natural pacemaker and usually fires at 60 to 100 beats per minute in a resting adult.
- Heart rate formula: heart rate = 60 / R-R interval in seconds.
- The P wave represents atrial depolarization, which leads to atrial contraction.
- The QRS complex represents ventricular depolarization, which leads to ventricular contraction.
- The PR interval is normally about 0.12 to 0.20 s and reflects conduction from the atria through the AV node to the ventricles.
Vocabulary
- Sinoatrial node
- The sinoatrial node is a cluster of pacemaker cells in the right atrium that normally starts each heartbeat.
- Atrioventricular node
- The atrioventricular node is a conduction relay between the atria and ventricles that briefly delays the electrical signal.
- Bundle of His
- The bundle of His is a pathway that carries impulses from the AV node into the interventricular septum.
- Purkinje fibers
- Purkinje fibers are fast-conducting fibers that spread the signal through the ventricular muscle so the ventricles contract together.
- Electrocardiogram
- An electrocardiogram is a recording of the heart’s electrical activity measured from electrodes on the skin.
Common Mistakes to Avoid
- Confusing electrical activity with blood flow is wrong because the conduction system sends signals through heart tissue, while blood moves through chambers and vessels.
- Thinking the AV node starts the normal heartbeat is wrong because the SA node is usually the primary pacemaker in a healthy resting heart.
- Assuming the P wave shows ventricular contraction is wrong because the P wave represents atrial depolarization, while the QRS complex represents ventricular depolarization.
- Ignoring the PR interval is wrong because an abnormally long PR interval can show delayed conduction through the AV node or nearby pathways.
Practice Questions
- 1 An EKG shows an R-R interval of 0.80 s. Calculate the heart rate in beats per minute using heart rate = 60 / R-R interval.
- 2 A patient’s PR interval is measured as 0.24 s. Is this within the normal 0.12 to 0.20 s range, and by how many seconds is it outside the nearest normal limit?
- 3 Explain why the AV node delay helps the heart pump more effectively before the ventricles contract.